1. Field of the Invention
The present invention relates to a semiconductor device having a light-receiving element, an optical pickup device and a method of manufacturing a semiconductor device having a light-receiving element.
2. Description of the Related Art
Photodiodes available as a light-receiving element capable of converting a light signal into an electrical signal are widely used as a control light sensor in a variety of photoelectric converters, e.g. a sensor for detecting an information signal (hereinafter referred to as an RF signal), a tracking error signal, a focusing error signal or the like in a so-called optical pickup device for optically recording or reproducing or for effecting both of optical recording and reproduction on an optical recording medium.
This light-receiving element is mounted on the same semiconductor substrate in a mixed state together with other circuit elements, e.g. a variety of circuit elements such as a bipolar transistor, a resistor, a capacitor or the like, thereby being configured as a so-called photo-IC (optical integrated circuit). Such photo-IC is generally manufactured in accordance with a manufacturing method of a bipolar transistor serving as the above-mentioned other circuit elements.
In a photo-IC having a high-speed and high-sensitivity light-receiving element, there is proposed such one having a high-resistance epitaxial semiconductor layer.
FIG. 1 is a schematic cross-sectional view of a conventional this kind of a photo-IC in which a photodiode PD serving as a light-receiving element and a bipolar transistor TR are mounted in a mixed state. In this example, there is configured a bipolar transistor available as a photo-IC in which an npn-type transistor TR and an anode-common type photodiode PD are formed on the same semiconductor substrate 1.
In this bipolar IC, a high impurity concentration p-type buried layer 3 is formed on the whole surface of one major surface of a p-type Si semiconductor base 2, and a low impurity concentration p-type first semiconductor layer 31 comprising an anode region 4 of the photodiode PD is epitaxially grown on this buried layer 3. Then, a high impurity concentration collector buried region 5 is formed on this first semiconductor layer 31 at its portion in which the transistor TR is formed. A high impurity concentration buried separating region 6 is selectively deposited between respective circuit elements and on the separating portion of the photodiode PD, which will be described later on, etc. Also, at the same time this buried separating region 6 is deposited, a p-type high impurity concentration buried region 8 is formed under a contact portion of an anode electrode 7 relative to the photodiode PD.
On the first semiconductor layer 31, there is further epitaxially deposited a low impurity concentration n-type second semiconductor layer 32 forming a cathode region 9 of the photodiode PD and a collector region 10 of the transistor TR.
On the surface of the Si semiconductor substrate 1 in which the first and second semiconductor layers 31 and 32 are epitaxially deposited on the semiconductor base 2, i.e. the second semiconductor layer 32, there is deposited a separating and insulating layer 11 made of SiO.sub.2 between electrically-separated semiconductor circuit elements or regions by a local heat oxidation, i.e. so-called LOCOS (Local Oxidation of Silicon).
A p-type high impurity concentration separating region 12 is formed between the separating and insulating layer 11 at the insulating and separating portion formed between the circuit elements below the separating and insulating layer 11 and the buried separating region 6 formed below the separating and insulating layer. A high impurity concentration p-type anode electrode deriving region 13 is deposited on the high impurity concentration buried layer 8, and a high impurity concentration anode contact region 14 is deposited on the anode electrode deriving region. A p-type high impurity concentration separating region 30 is deposited on the buried region 6 formed on the separated portion of the anode region 4 in contact with this region 6.
Then, an n-type high impurity concentration collector electrode deriving region 15 and a p-type base region 16 are deposited on the collector region 10. An n-type emitter region 17 is deposited on the base region 16.
Also, on each anode 4 of the photodiode PD, there is deposited a high impurity concentration cathode region 18 to which a cathode electrode 19 is contacted in an ohmic fashion.
On the surface of the semiconductor substrate 1, there is deposited an insulating layer 21 made of such as SiO2 or the like. This insulating layer has formed therethrough electrode contact-windows to which the emitter, the base and the collector electrodes 20E, 20B and 20C of the transistor TR are contacted, respectively. Then, on the insulating layer, there is deposited an interlayer insulator layer 22 such as SiO.sub.2 or the like. This interlayer insulator layer has formed thereon a light-shielding layer 23 made of Al or the like and having a light-receiving window. A protecting film 24 is deposited on this light-shielding layer.
Then, the insulating layers 21 and 22 are used as antireflection films so that a detection light is irradiated on the photodiode PD through the light-receiving window of the light-shielding layer 23.
The photodiode PD in the above-mentioned IC may be arranged as a sensor for detecting an RF signal, a tracking error signal and a focus error signal in an optical pickup device which is able to optically record, reproduce or both record and reproduce an optical recording medium, for example.
FIG. 2A shows a plan pattern view of the photodiode PD available as the sensor for detecting the RF signal, the tracking error signal and the focus error signal in the optical pickup device, for example. In this example, three light spots of a center light spot SP.sub.0 and side spots SP.sub.S1 and SP.sub.S2 on both sides from an optical recording medium, e.g. optical disk are irradiated on a quadrant photodiode PD.sub.0, for example, and photodiodes PD.sub.S1 and PD.sub.S2 on both sides, whereby the focus error signal is obtained by a calculation of (A+C)-(B+D) where A to D respectively represent outputs photo-electrically-converted at respective portions A, B, C and D of the quadrant photodiode PD, the tracking error signal is obtained by a calculation of (E-F) where E and F respectively represent the outputs of other two photodiodes PD.sub.S1 and PD.sub.S2 and the signal read-out output, i.e. RF signal is obtained by a calculation of (A+B+C+D).
FIG. 2B similarly shows an example of the photodiode PD which is applied to an optical pickup device, for example. In this example, spots SP.sub.1 and SP.sub.2 are irradiated on the photodiodes PD.sub.1 and PD.sub.2 which are respectively parallelly divided by four to provide photodiode portions A, B, C, D and A', B', C', D'. In this case, the two photodiode portions B, C and B', C' at the center of the respective photodiodes are formed as extremely-narrow stripe-like patterns of 14 (m pitch, for example. Then, when outputs of the respective portions A, B, C, D and A', B', C', D' of the respective photodiodes PD.sub.1 and PD.sub.2 are respectively assumed to be A, B, C, D and A', B', C', D', a focus error signal is obtained by a calculation of (B+C)-(A+D)-{(B'+C')-(A'+D')}, then a tracking error signal is obtained by a calculation of (A+B+C'+D')-(C+D+A'+B') and an RF signal is obtained by a calculation of (A+B+C+D)+(A'+B'+C'+D').
In the semiconductor device having the photodiode divided into a plurality of portions like the above-mentioned quadrant photodiode, as shown in FIG. 1, the cathode region 9 is separated along the whole thickness by the separating region 30 and the buried separating region 6 formed below the separating region.
That is, in the arrangement of the conventional semiconductor device, when the photodiode PD is not operated under the condition that the reverse bias voltage is not applied to the photodiode PD, the cathode region 9 is completely separated by the separating region 30 and the buried separating region 6 formed below the separating region. Then, when the photodiode PD is operated, by the reverse bias voltage applied to this photodiode, depletion layers are spread in the anode region 4 and the cathode region 9 from the p-n junction comprised of the anode region 4 and the cathode region 9 and the p-n junction comprised of the separating region 30 and the cathode region 9 as shown by dot-and-dash lines a and a' in FIG. 1.
However, when the semiconductor device including the light-receiving element formed of the above-mentioned arrangements of FIGS. 2A and 2B, i.e. so-called photo-IC is formed so that the light spots are irradiated over the respective portions of A, B, C, D and A', B', C', D', i.e. the respective separating region 30 and the buried separating region 6 formed below the separating region, a frequency characteristic of the photodiode is deteriorated.
The frequency characteristic in the photodiode is mainly determined by a CR time constant dependent on its parasitic capacity (C) and parasitic resistance (R), a time in which carriers travel within the depletion layer in the photodiode and a time in which carriers are diffused in the semiconductor layer which is not depleted.
Accordingly, in the above-mentioned quadrant photodiode, for example, the frequency characteristic changes at the position near the separating region 30 and the buried separating region 6 and at the position sufficiently distant from the above-mentioned position.
This will be described with reference to FIG. 1. In minority carriers, i.e. electrons e generated within the buried separating region 6 and the nearby anode region 4 by a light irradiation, the potential of this buried separating region 6 acts as a barrier against the electrons e of the minority carriers so that these electrons e are forced so as to move away from the depletion layer as shown by arrows b. As a result, these electrons e cannot linearly travel to the depletion layer and hence travel along the curved route. On the other hand, electrons e generated at the position sufficiently distant from the buried separating region 6 are not influenced or hardly influenced by this potential so that these electrons travel toward the depletion layer linearly as shown by arrows c. That is, the electrons generated in and near the buried separating region 6 should travel the long distance toward the depletion layer as compared with the electrons at the position sufficiently distant from the buried separating region 6. Therefore, a diffusion time of carriers is extended so that a frequency characteristic is deteriorated.
Therefore, as mentioned before, when the quadrant photodiode, for example, is used and light spots are irradiated on the regions including the buried separating region 6 and hence the separated regions, since the area of the separated regions occupies a large part of the light irradiated area, there arises a problem in the frequency characteristic. In particular, since the RF signal is detected as a sum signal from the respective separated regions, the deterioration of the frequency characteristic becomes a serious problem. Also, this becomes quite serious in the RF signal whose high speed efficiency is one of the most important requirements.